Hemispherical grain silicon (commonly referred to as HSG silicon) is one example of a crystallized material with a roughened surface, i.e., a surface that is not smooth. Electrically conductive roughened surfaces are useful in the manufacturing of dynamic semiconductor storage devices requiring storage node capacitor cell plates large enough to maintain adequate charge, i.e., capacitance, in the face of parasitic capacitances and noise that may be present during operation of a circuit including the storage devices. Maintaining storage node capacitance is especially important due to the continuing increases in Dynamic Random Access Memory (DRAM) array density.
Such DRAM devices, among others, rely on capacitance stored between two conductors separated by a layer of dielectric material. One method of increasing the capacitance of a capacitor formed using conductive polysilicon layers is to increase the surface area of the conductors. Using hemispherical grain (HSG) silicon for the first conductor is one method of increasing the surface area of the conductors because the later-deposited dielectric layer and second conductor will typically conform to the surface of the first deposited conductor.
Hemispherical grain silicon can be obtained by a number of methods including Low Pressure Chemical Vapor Deposition (LPCVD) at conditions resulting in a layer of roughened polysilicon. Another method includes depositing a layer of amorphous silicon, followed by high temperature seeding and/or annealing to cause the formation of hemispherical grain silicon.
The silicon layers to be converted into hemispherical grain silicon or deposited as hemispherical grain silicon are not, however, typically in situ doped during deposition because in situ doping of those layers can result in smaller grain size, thereby reducing surface area and capacitance. Furthermore, in processes where the HSG silicon layer is doped after formation, the roughness of the surface can be reduced by the post-formation doping processes. For example, diffusion doping of the HSG silicon may result in a layer of oxide that would need to be removed before deposition of the dielectric layer. The oxide layer may be removed using an etch process that could reduce the roughness of the HSG silicon layer. Ion implantation, another technique for doping an undoped layer of HSG silicon may also reduce the surface roughness of the HSG silicon.